A major design problem concerning the production of miniaturized DC—DC converter products is thermal management. More specifically, thermal impedance is a significant design consideration in the selection and design of a package for DC—DC converters. Materials that enable the level of circuit complexity and interconnect necessary to produce a functional package most often result in thermal impedances that make the package inoperable or impractical.
Die attach materials may include solder or adhesives like epoxies, with or without metal fillers like silver particles or flakes. The die attach material properties and geometry represent a significant factor in package resistance and reliability. Strain created in die attach materials is approximately proportional to the square root of the maximum linear dimensions of either the die or die attach area and is also approximately inversely proportional to the square root of die attach material bond line thickness (the thickness of the layer of adhesive material located between the semiconductor die and the package). Thus, to minimize strain in the die attach material, it is desirable to maximize the thickness of the die attach bond lines. However, thicker die attach bond lines result in higher thermal resistances and diminished product performance and reliability.
Efficient heat transfer between the semiconductor die and the die attach pad requires a thin bond material thickness while minimized die attach strain requires a thicker die attach material. The die attach material creates a thermal and electrical path between the semiconductor die and the die attach pad. Thus, increasing the thickness of the die attach material reduces the thermal and electrical performance of the die attach material by increasing the thermal and electrical resistance. Optimizing the heat transfer and strain characteristics of the die attach material present conflicting requirements. These two properties must be balanced in an effort to achieve maximum thermal and mechanical performance of the package.
Other problems associated with attaching large semiconductor dice in packages include poor adhesion of mold materials, such as epoxy Novalac, to metal surfaces and moisture diffusion into the package. Moisture turns to steam during normal reflow soldering operations and can break the bond between the mold material and metallization areas adjacent to or on the die attach area. High pressure steam in this area (exposed die attach pad) acts as a wedge during reflow soldering and can lead to increased moisture sensitivity for reflow solder processing and die attach separation.
Reduction of die attach stress encountered in assembly and end customer processing may be accomplished by controlling both the die attach material bond line thickness and reducing the amount of exposed die attach metallization to the mold material. At the same time, optimum thermal performance of the package is accomplished by controlling the die attach material bond line thickness.
Package mold materials have superior adhesion properties to solder mask materials used on laminate substrates than to exposed metallization like gold flash over nickel plated on copper traces which are commonly found on laminate substrates. Solder mask materials have excellent adhesion to those same metallization schemes. There are various substrate layout techniques for reducing or eliminating die attach metallization exposure to package mold material and controlling die attach material thickness.
FIG. 1 illustrates a technique that eliminates or reduces metallization exposure to package mold material by controlling the size of the solder mask opening. A solder mask 18 is formed over the substrate 12 such that it creates an opening or “window” that is slightly larger (e.g., 0.002 in) than the dimensions of the semiconductor die 10. A layer of die attach adhesive 14 is deposited into the opening and covers the die attach metallization layer 16. The semiconductor die 10 is placed into the solder mask opening and contacts the layer of die attach material 14. Since the solder mask opening is larger than the semiconductor die 10, a gap 20 is created between the edges 10a of the semiconductor die 10 and the edges 18a of the solder mask 18. The layer of die attach material 14 fills the solder mask “window” and the gap 20 when the semiconductor die 10 is pressed into the layer of die attach material 14. Thus, this technique eliminates possible metal exposure of the die attach metallization layer 16 to the mold material 22. The technique also creates a layer of die attach material 14 having a thickness T1 between the semiconductor die 10 and the die attach metallization layer 16. This technique is typically used when thin bond line control is required to maximize the thermal and electrical performance of the package.
FIG. 2 illustrates a technique to increase the die attach bond line material thickness. A semiconductor die 10 is attached to a substrate 12 by a die attach adhesive 14. In this technique, the solder mask 18 creates an opening or “window” that is slightly smaller (e.g., 0.002 in) than the dimensions of the semiconductor die 10. The die attach adhesive 14 is deposited into the solder mask opening and covers the die attach metallization layer 16. The semiconductor die 10 is placed over the solder mask opening and contacts the die attach material 14. The die attach material 14 fills the solder mask “window” when the semiconductor die 10 is pressed onto the die attach material 14. Since the solder mask opening is smaller than the semiconductor die 10, the edges 10a of the semiconductor die 10 slightly overlap the edges 18a of the solder mask 18. Thus, the semiconductor die 10 sits on a solder mask shelf 21 and eliminates possible exposure of the die attach metallization layer 16 to the package mold material 22. This technique creates a thicker layer of die attach adhesive 14, shown as T2 in FIG. 2, than the package shown in FIG. 1.
It would be desirable to reduce the die attach bond line material thickness without sacrificing the strain or stress properties of the package. It would also be desirable to reduce strain without sacrificing the thermal properties of the package.